1. Field of the Invention
The present invention relates to a glass-ceramic panel (or plate) intended in particular for covering or receiving heating elements, in particular intended to serve as a cook-top (or hob), and to its manufacturing process.
2. Description of the Related Art
The sales of glass-ceramic cook-tops have been continuing to grow over the last few years. This success is explained in particular by the attractive appearance of such cook-tops and by the ease of cleaning them.
It will be recalled that a glass-ceramic is originally a glass, called precursor glass, the chemical composition of which allows controlled crystallization to be induced by suitable heat treatments, called ceramification. This partly crystallized specific structure gives the glass-ceramic unique properties.
At the present time, there are various types of glass-ceramic panels, each variant being the result of extensive research and many tests, given that it is very difficult to make modifications to these panels and/or to the process for obtaining them without risking an unfavourable effect on the desired properties. To be able to be used as a cook-top, a glass-ceramic panel must generally have a transmission in the wavelengths in the visible range that is both low enough to mask at least some of the subjacent heating elements when not in use and high enough so that, for the sake of safety, the user can visually detect the heating elements that are in operation. It must also have a high transmission in the wavelengths of the infrared range.
Most current panels are of dark color, in particular black, but there are also panels of lighter color (in particular white or cream color having, for example, a haze of at least 50%, as described in patent FR 2 766 816), or even transparent panels provided with opacifying coatings. Among known coatings for glass-ceramic panels are in particular enamels or paints, which coatings may be, depending on the case, on the upper face (in the use position) and/or lower face of the panel, and it is possible for these coatings to be of decorative and/or functional character, for example able to represent logos and/or to delimit heating regions, and/or able to mask underlying elements (underlying metal structures or heating elements), etc.
The advantages of coatings on the lower face (or more generally on the unexposed face, after the panel has been mounted in the use position) are especially better protection of these coatings from soiling and abrasion (mainly undergone by the exposed face of the panel), easier manufacture and handling (in particular when all the coatings are on this same face) and improved comfort for the user (easier cleaning of the exposed face). However, these coatings also have drawbacks such as greater risk of thermal degradation near the heating elements, or certain irksome optical effects visible on the exposed face (as the case may be, double images, appearance and/or contrast variations depending on the closeness of the underlying elements and/or on the border, iridescence phenomena whereby the color appears different depending on the angle at which the panel is observed or depending on the angle of illumination of the panel, etc.).
In conventional dark-colored panels, it is known to deposit, on the lower face, a dark (black or dark grey) coating forming a screen for concealing, for example, the framework of the structure (such as a cooker) or the frame or sections on which the panel has to be mounted. Although this use is widespread and satisfactory for dark panels, it is however not very appropriate for light-colored panels on which unattractive effects and reflections are more easily noted (in particular iridescence phenomena or color differences appearing on the upper face between the coated part and the uncoated part) partly due to this screen. Another solution consists in mounting a dark opacifying intermediate element (such as an aluminium foil or intermediate aluminium parts), but this requires the addition of a further element and may also generate unattractive effects and reflections.